Learn how to set up a grow room for marijuana, including adjusting the climate, air flow, grow lights, and watering and fertilization systems. A Step-By-Step Guide to Growing Marijuana Indoors
How to Grow Cannabis Indoors
In terms of temperature, remember this rule: No colder than 60, no hotter than 85, and never above 90.
Focus on air flow
- Help to regulate heat and humidity: Grow lights kick out a lot of heat, which also increases the humidity in the grow room. An exhaust fan pulls hot and humid air out of the grow room, creating a vacuum that pulls in cooler, drier air (assuming the room has intake holes or vents).
- Deliver CO2 to plants: Plants breathe in carbon dioxide (CO2) and breathe out oxygen (O2). Without proper ventilation, the CO2 supply in the room is depleted, and the plants “suffocate.”
- Prevent pests and diseases: Warm, humid, stagnant air provides an ideal environment for mold, mildew, fungi, and certain pests. Pulling in cooler, drier air eliminates this problem, and having a breeze in the room helps to discourage infestations of small flying insects such as gnats.
- Strengthen plant stalks and stems: Plants sense the breeze in the room and grow hardier as a result, which provides more support for buds during the flower stage.
Improper air flow in grow rooms is the number one reason for reduced yields and complete crop failure.
Ensure proper ventilation
The first order of business is to install one or two fans to ventilate the room — an exhaust fan, an intake fan, or both. With an active system, you have an exhaust fan on one end of the room and an intake fan of the same size on the opposite end. In a passive system, you use only one fan. As the exhaust fan pulls air out of the room or the intake fan pushes air into the room, air flows in or out through one or more holes on the opposite end of the room. In passive systems, the hole (or holes) without the fan must be larger than the hole with the fan.
Most grow rooms use in-line duct fans, which are very easy to install. Installation is similar to connecting a flexible duct pipe to a clothes dryer. You can buy 4-, 6-, or 8-inch diameter in-line duct fans depending on the size of the room and the size of any existing holes. Six-inch fans are common. If you have a grow tent, check the size of the exhaust and intake holes and buy fans to match.
Also check the cubic feet per minute (CFM) rating of the fan(s) and buy a fan with a CFM rating that’s higher than the volume of the room in cubic feet. The general idea is that you want sufficient ventilation to completely replace the air in the grow room once every minute. Simply measure the room’s length, width, and height in feet and multiply the three numbers. For example, if the room is 3-by-3-by-6 feet, 3 × 3 × 6 = 54 cubic feet, so a fan with a rating of 100 CFM would be sufficient. However, you may need a fan with a higher CFM rating if you’re pumping the air over a long distance or have one or more bends in the duct pipe.
Your intake hole should be near the bottom at one end of the room with the exhaust hole at the top of the opposite end of the room. The exhaust hole is higher, because heat naturally rises to the top.
Whether you use one or two fans, install a filter on the intake and exhaust ducts. The intake filter keeps out bugs, mold spores, dust, and other contaminants. The exhaust filter is usually a carbon filter that helps to reduce the odors from the cannabis exiting the room. You attach the filters directly to the fans or use a piece of flexible duct pipe between the fan and filter.
Use as little duct pipe as necessary and run it as straight as possible. The longer the distance the air has to travel and the more bends in the pipe, the less efficient the air flow. If you must run pipe a long distance or add a bend, consider buying fans with higher CFM ratings.
Circulate the air
Air circulation is also important. Plants don’t “exhale” with any type of force during respiration. Fans used to circulate the air move the O2 surrounding the plants and replace it with CO2 that the plants can “breathe in.” You need one or more fans inside your grow room to maintain proper circulation. Deciding on the number of fans and positioning them in the room is mostly a process of trial and error. The goal is to have all parts of all plants “dancing” — all the leaves should be shaking gently. If you notice any part of any plant that’s not dancing, you may need to reposition the fan(s) or add a fan.
Start with two small fans in opposite corners of the room or one slightly larger oscillating fan in one corner of the room and make adjustments from there.
Supply carbon dioxide
Plants require CO2 to survive. This is the symbiotic relationship plants have with animals. Animals breath in O2 and exhale CO2; plants “inhale” CO2 and “exhale” O2. If your grow room has adequate air flow, CO2 sublimation isn’t necessary, but it increases overall yields if you’re using higher intensity lighting.
Several methods are available for adding CO2 to a grow room. You can buy a tank of CO2 and simply pump it into the room, allow dry ice to melt inside the room, or buy CO2 canisters or bags that release the gas slowly into the room over a period of time. If you’re adding CO2 to your grow room, keep the following important points in mind:
- Add CO2 only when the lights are on. When the lights are off, plants slow down their use of CO2 considerably, so any CO2 added is CO2
- Turn off the intake and exhaust fans for a few minutes when releasing CO2; otherwise, you’re pumping out the gas, and wasting it.
- Add CO2 from the top of the room and in front of one of your circulating fans. It’s denser than air, so it tends to drop toward the floor. For example, if you’re using a CO2 tank, run a hose to near the top of the grow room and lower it so it’s in front of one of the fans.
- Maintain a level of 900 parts per million (ppm) of CO2 during the vegetative stage and 1,150 ppm during the flower stage. You’ll need a CO2 meter to monitor CO2
Additional CO2 is necessary with higher light intensity, enabling the plant to take advantage of the added light with greater photosynthesis.
Set up grow lights
Lighting is a key factor in a successful indoor grow operation. The types of lights, the way you set them up, and other pieces that control and direct them are the keys to your yield and the flavor of your end product. Here, we guide you through the process of choosing and installing your grow lights.
Calculate your lighting needs
Before you head out to your local nursery or hardware store to shop for grow lights figure out how much light you need. In general, a standard 1,000 watt grow light will cover four plants that have a fully grown diameter of about 3 feet, depending on strain. If you set up your grow lights and plants and notice that some parts of one or more plants aren’t receiving light, you’ll need to add one or more lights.
Choose light fixtures and bulbs
Most standard household light fixtures and bulbs are insufficient for growing cannabis. They don’t provide the intensity and quality of light the plants need for optimal growth. The exception is fluorescent lights (typically T5s) or compact fluorescent lights (CFLs), which are okay, but result in smaller, lower-quality buds. We don’t recommend fluorescent lighting.
After ruling out fluorescent lighting, your choice of grow lights depends on your goal and the stage of growth:
- If your goal is high yields, choose a high intensity discharge (HID) bulb — metal-halide (MH) for the vegetative stage and high-pressure sodium (HPS) during the flower stage. These bulbs emit a lot of light and a lot of heat, so you need to position them at a greater distance from the plants.
- If you’re looking for better terpene yields for extraction, use light-emitting diode (LED) or ceramic metal halide (CMH) bulbs, because these preserve the terpenes without bulking up the flower weight and density the way high-intensity discharge (HID) lighting does.
- Fixture with reflector hood: The fixture holds the bulbs, and the reflector hood directs the light down to the plants. Reflector hoods come in different types:
- Closed hood: Shaped like a box, a closed hood reflector creates a more focused beam of light (and heat).
- Vented hood: Similar to a closed hood reflector but with openings on the ends for connecting the hood to in-line duct fans for cooling.
- Wing: Typically a curved and textured aluminum sheet that provides a less focused beam of light than a closed hood reflector. The light covers a greater area but is less intense (so is the heat).
- Parabolic: Shaped like an umbrella, a parabolic hood distributes light like a wing but in a more circular pattern.
Your choice of hoods is a personal preference. Go with a closed hood if you’re concerned about heat or with an wing or parabolic if you’re not.
- Ballast: The ballast provides control over the current that the lightbulb draws from the power source. The following two types of ballasts are most common:
- Magnetic: Less expensive, heavy, hot, potentially noisy, susceptible to flicker, and supports only bulbs of a certain wattage. If you want to change from a 400W bulb to a 600W bulb, for example, you need to replace the ballast.
- Digital: More expensive, smaller, lighter, cooler, quieter, less susceptible to flicker, more efficient, may be equipped with a dimmable option, may cause radio frequency interference.
- Hooks and pulleys: Grow light systems often include hooks and pulleys for hanging the light fixtures in your grow room. Pulleys enable you to more easily raise and lower the light fixtures to place them at the right distance from the tops of the plants.
- Timer: Grow light systems typically come with a timer, or you can purchase a timer separately, which automates the process of cycling the lights on and off on schedule.
Mount your light fixtures
Mount the light fixtures to the ceiling of the grow room above the plants, positioning the fixtures to ensure equal distribution of light over the entire canopy. How you mount the light fixtures depends on the fixture and how your grow room ceiling is configured. Using hooks, ropes, or chains and possibly pulleys, you can hang your fixtures in a way that you can easily raise and lower them to the proper distance from the tops of your plants.
Position the lights above the plants, so all parts of all plants are receiving light. The light should be as close to the top of the tallest plant as possible without burning it. Keep a close eye on the plants whenever adjusting the lights, and if the top of any plant is getting burned, raise the light.
Don’t place anything flammable close enough to the light that there’s any possibility the light will ignite it.
Set and reset timers
During the vegetative stage, plants require 18 to 24 hours of light. During the bloom/flower stage, they need 10 to 12 hours of light and at least 12 hours of total darkness (for photoperiod strains); auto-flowering strains will flower without 12 hours of darkness. Putting your grow lights on timers greatly simplifies the process of managing the required light/dark cycles, but you still need to manage the changes in lighting over the growth cycle.
If you plan to have a continuous garden with some plants in veg and some in bloom, set up your lighting differently in those two areas. For photoperiod strains, use a separate grow tent or grow room for plants that are in the vegetative stage and those that are in the flower stage.
- Position the lights at the proper distance above the canopy for the vegetative stage.
- Adjust your light timer(s) to provide 18 to 24 hours of light. Experiment with different settings in that range over several grows to find the optimum amount of light for each strain you grow.
- Keep an eye on your plants, adjusting the lighting as necessary to keep the lights the proper distance from the tops of the plants as they grow taller.
When your plants are about half the size of full-grown plants, they’re ready to switch from the vegetative to the flower stage. (At this point, you either adjust the lighting, as explained in the remaining steps or move the plants to the flower tent or room.)
The size of a full-grown plant is strain dependent and impacted by light, container size, and other environmental influencers such as CO2. You may have to go through several rounds of growing a particular strain to develop a clear idea what the size of a full-grown plant is and when the plant is ready to switch from the vegetative to the flower stage.
You don’t need to change out fluorescent, CFL, or LED bulbs.
When changing to the brighter HPS bulbs, shade the plants for a couple days to prevent them from getting blasted by the more intense light. You can place a piece of cardboard between the light and the plants to serve this purpose, but make sure it’s as far as possible from the light to prevent a fire.
Measure the light
Light intensity has a big impact on yield. All parts of all plants should have exposure to the light, and the lights should be as close to the plants as possible without burning them. If the top of any plants are wilting or burnt from the light, raise the lights.
For more sophisticated grows, obtain a photosynthetic active radiation (PAR) meter and take measurements at several different locations above the canopy to measure the PAR output of the lights. The PAR measure should never rise above 1,200 PAR.
Decide on a watering/fertilizing system
Whether you’re growing indoors or outdoors, you need to decide on a system for watering and fertilizing your plants. You basically have two options: manual and automatic. During your first grows, we recommend the manual method as you develop a sense of how much water and fertilizer your plants generally need.
After developing an understanding of your plants’ water and nutrient needs (which may vary depending on the strain), consider installing an automated irrigation system. These systems are equipped with timers that water and feed plants automatically on a pre-set schedule. They provide the same benefits of lighting systems — the convenience and reliability of automation. However, you still need to monitor your plants to be sure they’re getting enough and not too much water and nutrients.
Use a hydroponics system
- Aeroponic: Plants sit in a tray above a water/nutrient reservoir with their roots dangling down. Solution from the reservoir is sprayed up onto the roots at regular intervals, and excess solution drips down into the reservoir.
- Drip: Nutrient-rich water is dripped slowly at regular intervals into the grow medium where the roots can absorb it. Unused water drains back to the reservoir to be reused or to a waste reservoir and then discarded.
- Deep water culture (DWC): Plants sit in baskets above an aerated (and typically chilled) water/nutrient reservoir with their roots submerged in the solution, which allows for continuous feeding.
- Ebb and flow: Plants sit in pots in a grow tray. Nutrient-rich water is pumped into the grow tray at regular intervals and flows into holes at the bottom and sides of the pots. The pumping stops and water is allowed to drain back into the reservoir from which it was pumped.
- Nutrient film technique (NFT): NFT is like a cross between DWC and ebb and flow. Plants sit in baskets above a grow tray. Nutrient-rich water is continuously pumped from a reservoir into the grow tray and then drains from the opposite end of the grow tray back into the reservoir. This arrangement delivers a continuous flow of nutrient-rich water to the roots.
- Wick: A plant sits in a container above an aerated, nutrient-rich water reservoir, and a rope or other absorbent material (such as felt) is placed through the middle of the growth medium and into the reservoir. Through capillary action, the solution from the reservoir “climbs” the rope, providing the plant with as much or as little water and nutrients as it demands.
Here are a few suggestions for increasing your odds of a successful hydroponics grow:
- Disinfect all your hydroponics equipment with isopropyl alcohol or bleach between grows to kill off any bacteria or other infectious agents. Anaerobic bacteria can build up in dirty systems and kill your plants from the roots up.
- Use clean, pH neutral water. Water from a reverse osmosis (RO) system or distilled water is suitable.
- Aerate the nutrient-rich water solution. You can place an aeration stone in the bottom of the reservoir attached to a small air pump like those carried by local pet stores. Without aeration, your plants may not receive the oxygen they need.
- Replace the water/nutrient solution every couple weeks. Don’t merely add nutrients, because nutrient concentrations may become too high as a result. (Remember to use a fertilizer with a higher nitrogen concentration during the vegetative stage and higher potassium and phosphorous during the flower stage.)
- After dumping the old nutrient solution, run a dilute water and hydrogen peroxide solution through the system to clear out any infectious agents and then rinse with plain water.
- Consider flushing the grow medium with plain water whenever you change the nutrient solution.
When choosing and setting up a hydroponics system, research to find out the type of system that’s best for your grow space and skill level. Simpler is usually better. Use high quality food grade plastics in your system and make sure it’s leak free before starting your grow.
Keep your grow room impeccably clean
- After each use, wash and disinfect plant containers, grow trays, irrigation hoses, and pumps. Use soap and water followed by isopropyl alcohol or a bleach solution (one part bleach to three parts water). Then, carefully rinse everything with plain water.
- Keep your grow room free of any dead plant mater and debris. This is where many pests and pathogens can get a foothold in a garden of healthy plants.
- Watch for common pests such as aphids, fungus gnats, spider mites, and thrips. If you see even one of these nasty critters, identify it and find an effective pesticide. This is where your friendly garden store or grow store staff comes in handy.
About This Article
This article is from the book:
About the book authors:
Kim Ronkin Casey has been a communications professional for more than 20 years and recently took a year-long leap into the world of cannabis as the communications manager for one of the leading dispensaries in North America. She now consults for companies in the industry on internal and external communications. Joe Kraynak is a professional writer who has contributed to numerous For Dummies books.
Growing Cannabis From Seeds Indoors
Article written by
Dipak Hemraj Head of Research and Education
Dipak Hemraj is a published author, grower, product maker, and Leafwell’s resident cannabis expert. From botany & horticulture to culture and economics, he wishes to help educate the public on why cannabis is medicine (or a “pharmacy in a plant”) and how it can be used to treat a plethora of health problems. Dipak wants to unlock the power of the plant, and see if there are specific cannabinoid-terpene-flavonoid profiles suitable for different conditions.
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